Strong nonlinear selection against fluctuating asymmetry in wild populations of a marine fish

Microbiome and epigenetic variation in wild fish with low genetic diversity

Non-genetic sources of phenotypic variation, such as the epigenome and the microbiome, could be important contributors to adaptive variation for species with low genetic diversity. However, little is known about the complex interaction between these factors and the genetic diversity of the host, particularly in wild populations. Here, we examine the skin microbiome composition of two closely-related mangrove killifish species with different mating systems (self-fertilising and outcrossing) under sympatric and allopatric conditions. This allows us to partition the influence of the genotype and the environment on their microbiome and (previously described) epigenetic profiles. We find the diversity and community composition of the skin microbiome are strongly shaped by the environment and, to a lesser extent, by species-specific influences. Heterozygosity and microbiome alpha diversity, but not epigenetic variation, are associated with the fluctuating asymmetry of traits related to performance (vision) and behaviour (aggression). Our study identifies that a proportion of the epigenetic diversity and microbiome differentiation is unrelated to genetic variation, and we find evidence for an associative relationship between microbiome and epigenetic diversity in these wild populations. This suggests that both mechanisms could potentially contribute to variation in species with low genetic diversity.

Signatures of increasing environmental stress in bumblebee wings over the past century: Insights from museum specimens

Determining when animal populations have experienced stress in the past is fundamental to understanding how risk factors drive contemporary and future species' responses to environmental change. For insects, quantifying stress and associating it with environmental factors has been challenging due to a paucity of time-series data and because detectable population-level responses can show varying lag effects. One solution is to leverage historic entomological specimens to detect morphological proxies of stress experienced at the time stressors emerged, allowing us to more accurately determine population responses. Here we studied specimens of four bumblebee species, an invaluable group of insect pollinators, from five museums collected across Britain over the 20th century. We calculated the degree of fluctuating asymmetry (FA; random deviations from bilateral symmetry) between the right and left forewings as a potential proxy of developmental stress. We: (a) investigated whether baseline FA levels vary between species, and how this compares between the first and second half of the century; (b) determined the extent of FA change over the century in the four bumblebee species, and whether this followed a linear or nonlinear trend; (c) tested which annual climatic conditions correlated with increased FA in bumblebees. Species differed in their baseline FA, with FA being higher in the two species that have recently expanded their ranges in Britain. Overall, FA significantly increased over the century but followed a nonlinear trend, with the increase starting c. 1925. We found relatively warm and wet years were associated with higher FA. Collectively our findings show that FA in bumblebees increased over the 20th century and under weather conditions that will likely increase in frequency with climate change. By plotting FA trends and quantifying the contribution of annual climate conditions on past populations, we provide an important step towards improving our understanding of how environmental factors could impact future populations of wild beneficial insects.

Fluctuating Asymmetry in the Polymorphic Sand Cricket (Gryllus firmus): Are More Functionally Important Structures Always More Symmetric?

Simple Summary

Asymmetry in bilateral structures occurs when animals experience perturbations during development. This fluctuating asymmetry (FA) may serve as a reliable indicator of the functional importance of a structure. For example, locomotor structures often display lower levels of FA than other paired structures, highlighting that selection can maintain symmetry in traits important for survival or reproduction. Species that have multiple distinct morphs with unique behaviors and morphologies represent an attractive model for studying the relationship between symmetry and function. The sand field cricket (Gryllus firmus) has two separate morphs that allow us to directly test whether individuals maintain higher levels of symmetry in the structures most vital for maximizing fitness based on their specific life strategy. Longwing (LW) individuals can fly but postpone reproduction until after a dispersal event, whereas shortwing (SW) individuals cannot fly but begin reproducing in early adulthood. We quantified FA across a suite of key morphological structures indicative of investment in growth, reproduction, and flight capability for males and females across the morphs. Although we did not find significant differences in FA across traits, as predicted, locomotor compensation strategies may reduce selective pressures on symmetry or developmental patterns may limit the optimization between trait form and function.

Abstract

Fluctuating asymmetry (FA) may serve as a reliable indicator of the functional importance of structures within an organism. Primary locomotor structures often display lower levels of FA than other paired structures, highlighting that selection can maintain symmetry in fitness-enhancing traits. Polyphenic species represent an attractive model for studying the fine-scale relationship between trait form and function, because multiple morphs exhibit unique life history adaptations that rely on different traits to maximize fitness. Here, we investigated whether individuals of the wing polyphenic sand field cricket (Gryllus firmus) maintain higher levels of symmetry in the bilateral structures most vital for maximizing fitness based on their specific life history strategy. We quantified FA and directional asymmetry (DA) across a suite of key morphological structures indicative of investment in somatic growth, reproduction, and flight capability for males and females across the flight-capable longwing (LW) and flight-incapable shortwing (SW) morphs. Although we did not find significant differences in FA across traits, hindwings lacked DA that was found in all other structures. We predicted that functionally important traits should maintain a higher level of symmetry; however, locomotor compensation strategies may reduce the selective pressures on symmetry or developmental constraints may limit the optimization between trait form and function.

Radiocaesium accumulation and fluctuating asymmetry in the Japanese mitten crab, Eriocheir japonica, along a gradient of radionuclide contamination at Fukushima

The 2011 Tohoku earthquake-tsunami and the subsequent nuclear accident at the Fukushima Dai-ichi Nuclear Power Station (FDNPS) led to large-scale radionuclide contamination of the marine and freshwater environment. Monitoring studies of marine food products in the Fukushima region have generally demonstrated a declining trend in radiocaesium concentrations. However, the accumulation and elimination of radiocaesium and potential biological effects remain poorly understood for freshwater biota inhabiting highly contaminated areas at Fukushima. Consequently, the present study aimed to assess radiocaesium accumulation and developmental effects on the commercially important catadromous Japanese mitten crab, Eriocheir japonica. E. japonica were collected from four sites along a gradient of radionuclide contamination 4-44 km in distance from the FDNPS in 2017. To determine potential developmental effects, fluctuating asymmetry (FA) was used as a measure of developmental stability. Combined ¹³⁴Cs and ¹³⁷Cs values for whole E. japonica from highly contaminated sites 4 and 16 km in distance from the FDNPS were 3040 ± 521 and 2250 ± 908 Bq kg^-1 wet weight respectively, 30 and 22 times greater than the Japanese standard limit of 100 Bq kg^-1. Estimated total dose rates based on radiocaesium concentrations in whole crabs and sediment ranged from 0.016 to 37.7 μGy h^-1. No significant relationship between radiocaesium accumulation and FA was recorded, suggesting that chronic radiation exposure at Fukushima is not inducing developmental effects in E. japonica as measured using fluctuating asymmetry. Furthermore, estimated dose rates were below proposed regulatory limits where significant deleterious effects are expected. The present study will aid in the understanding of the long-term consequences of radiation exposure for non-human biota and the management of radioactively contaminated environments.

The influence of heavy metals on the shape and asymmetry of wings of female Polistes nimpha (Hymenoptera, Vespidae) living on contaminated sites

The aim of the present study was to determine the fluctuating asymmetry of the first pair of wings in females Polistes nimpha (Christ, 1791) living in an environment contaminated with heavy metals. The average concentration of Zn, Cd and Pb in the bodies of the insects varied depending on the distance from the source of contamination, reaching the highest values on the site closest to the source of contamination and the lowest at the most distant site. As a result of the morphometric analyses, significant differences were found in the asymmetry values of the first pair of wings depending on the level of Zn, Cd, Pb accumulated by the wasps. In the case of shape asymmetry, differences were found for all the effects studied (year of capture and site). Significant differences were also found in the size of wings between individuals captured on Sites 1 and 2 and those caught on Site 3. Specimens caught on site characterized by the lowest concentration of heavy metals in the topsoil, proved to be significantly larger than the insects collected on the other sites. There were no differences in the size of individuals between the different years of capture. Based on the results obtained by us, it can be assumed that the wings of P. nimpha females may become a useful object in studying the impact of environmental stress of Zn, Cd and Pb pollution on the symmetry of their wings.

Prenatal Developmental Trajectories of Fluctuating Asymmetry in Bat Humeri

Fluctuating asymmetry (random fluctuations between the left and right sides of the body) has been interpreted as an index to quantify both the developmental instabilities and homeostatic capabilities of organisms, linking the phenotypic and genotypic aspects of morphogenesis. However, studying the ontogenesis of fluctuating asymmetry has been limited to mostly model organisms in postnatal stages, missing prenatal trajectories of asymmetry that could better elucidate decoupled developmental pathways controlling symmetric bone elongation and thickening. In this study, we quantified the presence and magnitude of asymmetry during the prenatal development of bats, focusing on the humerus, a highly specialized bone adapted in bats to perform under multiple functional demands. We deconstructed levels of asymmetry by measuring the longitudinal and cross-sectional asymmetry of the humerus using a combination of linear measurements and geometric morphometrics. We tested the presence of different types of asymmetry and calculated the magnitude of size-controlled fluctuating asymmetry to assess developmental instability. Statistical support for the presence of fluctuating asymmetry was found for both longitudinal and cross-sectional asymmetry, explaining on average 16% of asymmetric variation. Significant directional asymmetry accounted for less than 6.6% of asymmetric variation. Both measures of fluctuating asymmetry remained relatively stable throughout ontogeny, but cross-sectional asymmetry was significantly different across developmental stages. Finally, we did not find a correspondence between developmental patterns of longitudinal and cross-sectional asymmetry, indicating that processes promoting symmetrical bone elongation and thickening work independently. We suggest various functional pressures linked to newborn bats’ ecology associated with longitudinal (altricial flight capabilities) and cross-sectional (precocial clinging ability) developmental asymmetry differentially. We hypothesize that stable magnitudes of fluctuating asymmetry across development could indicate the presence of developmental mechanisms buffering developmental instability.

Patterns of fluctuating asymmetry in the limbs of freshwater turtles: Are more functionally important limbs more symmetrical?

Understanding how selective forces influence patterns of symmetry remains an active area of research in evolutionary biology. One hypothesis, which has received relatively little attention, suggests that the functional importance of morphological characters may influence patterns of symmetry. Specifically, it posits that for structures that display bilateral symmetry, those with greater functional importance should display lower levels of asymmetry. The aim of this study was to examine the patterns of fluctuating asymmetry (FA) present in the limb bones of freshwater turtles in the family Emydidae. Aquatic emydid turtles of the subfamily Deirochelyinae employ a hindlimb-dominant swimming style, suggesting that hindlimbs should display lower levels of FA. Consistent with the morpho-functional hypothesis of symmetry, we found a strong, clade-wise pattern of humeral-biased FA in aquatic Deirochelyinae. In contrast, some emydids of the subfamily Emydinae possess more terrestrial tendencies. As terrestrial locomotion places more equal importance on fore- and hindlimbs, we predicted that such behaviors may minimize differences in FA. No clade-wise pattern was detected in the subfamily Emydinae. We also detected a phylogenetic signal in FA within the femur and discovered that FA has evolved at vastly different rates between the fore- and hindlimbs.

Patterns of fluctuating asymmetry in the limbs of anurans

It has been hypothesized that fluctuating asymmetry (FA) may provide an indication of the functional importance of structures within an organism, with structures that more strongly impact fitness being more symmetric. Based on this idea, we predicted that for tetrapods in which the forelimbs and hindlimbs play an unequal role in locomotion, the less functionally important limb set should display higher levels of FA. We conducted a multispecies test of this hypothesis in anurans (frogs and toads), whose saltatory locomotor mode is powered by the hindlimbs. We also tested whether FA in the forelimbs, which play a more important role during landing, differed between families that differ in the degree of forelimb use in locomotion (Bufonidae vs. Ranidae). We calculated FA from the lengths of humeri and femora measured from disarticulated skeletal specimens of four anuran taxa (Bufonidae: Anaxyrus americanus, Rhinella marina; Ranidae: Lithobates catesbeianus, Lithobates clamitans). Our findings were consistent with the hypothesis that natural selection for increased locomotor performance may influence patterns of FA seen in vertebrate limbs, with all species displaying lower mean FA in the hindlimbs. More subtle functional roles between the forelimbs of bufonids and ranids, however, did not elicit different levels of FA.

Developmental Costs of Biological Invasion: The Exotic Wood Borer Tetropium fuscum (Coleoptera: Cerambycidae) is More Asymmetric and Smaller in Invaded Area

Biological invasions provide a unique opportunity to gain insight into basic biological processes occurring under new circumstances. During the process of establishment, exotic species are exposed to various stressors which may affect their development. Presence of the stressors is often detected by measurements of left-right body asymmetry, which consists of two main components: fluctuating asymmetry and directional asymmetry. Fluctuating asymmetry constitutes random differences between the two body sides, whereas directional asymmetry occurs when a particular trait is bigger on one of the sides. The relation between these two asymmetry components is still not fully understood. Our goal was to investigate the potential differences in asymmetry patterns between native and invasive populations of Tetropium fuscum (Fabr. 1787) (Coleoptera: Cerambycidae), a harmful forest pest native to Europe and introduced to North America. Wing asymmetry assessment was based on the geometric morphometrics of hind wings. We found that specimens from invaded area were markedly smaller and have more asymmetric wings than individuals from native population, suggesting some unfavorable conditions in the invaded area. Moreover, we found significant directional asymmetry in the native but not in the invasive population. On the other hand, differences between left and right hind wings were similar in the native and invasive populations, in terms of direction. This suggests that a high level of fluctuating asymmetry in the invasive population may blur the intrinsic directional asymmetry and hinder its detection. Our data show that fluctuating asymmetry has a potential as an indicator of developmental stress in invasive species.